Methods for determining a contrast agent concentration in the body material of a human or animal patient and for simultaneously differentiating between two different tissue types are widely known. In particular, the following method variants are used in this process:
Previously, two different techniques were known for determining a contrast agent concentration using computed tomography (CT). In the first technique, a computed tomography image of the body region in which the contrast agent concentration is intended to be measured can in each case be recorded before and after contrast agent is administered. After registering the two CT images obtained in the process, they are subtracted from one another in order to obtain the increased X-ray attenuation values caused by the contrast agent for every pixel or voxel. This increase in the X-ray attenuation values is pro-portional to the concentration of the contrast agent. However, as a result of this requiring the computed tomography images at different times, registration and/or movement artifacts can occur and can lead to an erroneous determination. Furthermore, if a contrast agent is used which only accumulates slowly in the body material, an undesirably long waiting period has to be observed between the two computed tomography images.
The second known technique utilizes the use of a multi-energy computed tomography scanner in order to record, simultaneously, two computed tomography images with different spectral distribution of the X-ray radiation, i.e. with different X-ray energy. In a variant of this technique, the image data records for both X-ray energies are first of all reconstructed separately from each other. Subsequently, the measured X-ray attenuation values for each voxel are decomposed into the molecular density of two base materials (2 material decomposition), the contrast agent constituting one base material. The two equations resulting from the decomposition can be used to determine the two unknowns for each voxel: the concentrations of the two base materials. However, this technique does not supply satisfactory results for a number of body materials because the decomposition for all material components comprised in the body material is not readily foreseeable. Thus, the application of this technique for determining the contrast agent concentration in the liver (which generally also contains a relatively large proportion of fat) results in a mixture of the two base materials which is difficult to interpret.
Furthermore, reference is made to the applicant's patent application with the file reference DE102006009222.8, which is not a prior publication and makes possible a 3 material decomposition of an examined region using a dual energy CT examination. Herein, the region under examination, preferably a human or animal patient, is subdivided into two different tissue types and the quantitative occurrence of a contrast agent is determined at the same time.
In this last-mentioned method, two computed tomography images of the body material are recorded using a multi-energy computed tomography scanner, in particular a so-called dual energy computed tomography scanner, at two different spectral distributions of the X-ray radiation. Recording using the two different X-ray energies is preferably performed simultaneously in this case. Two image data records which contain X-ray attenuation values x are reconstructed in a known fashion from the measurement data of the computed tomography images. Here, X-ray attenuation values can be understood to be both the attenuation coefficients μ and values, such as the CT value, derived therefrom.
In the present method, the X-ray attenuation values x for each voxel of interest in the two image data records are decomposed into X-ray attenuation values of three material components. These three material components are the two different material components of the body material and the substance whose concentration is intended to be determined. It goes without saying that the two different material components of the body material do not have to be chemically pure materials, but they can also constitute material mixtures. In the present method, the X-ray attenuation values are decomposed under the assumption that the X-ray attenuation value xM of the body material M without the substance is made up of the X-ray attenuation values xM1, xM2 of the first and second material component according to the following equation:xM=f*xM1+(1−f)*xM2,where f corresponds to a volume proportion of the first material component in the body material. The concentration of the substance in each voxel of interest is then determined on the basis of this decomposition. This is possible because for each voxel there are respectively two equations corresponding to the two image data records with a total of two unknowns: the volume proportion f of the first material component and the concentration c of the substance accumulated in the body material.
According to one refinement of this method, the concentration of the substance is therefore also determined by the solution of this system of equations comprising the following two equations:xE1=c*xKM,E1+f*xM1,E1+(1−f)*xM2,E1 xE2=c*xKM,E2+f*xM1,E2+(1−f)*xM2,E2 where xE1/E2 corresponds to the X-ray attenuation values in the two image data records at the different spectral energy distributions or energies E1 and E2 of the X-ray radiation and c corresponds to the concentration of the substance in the body material. The X-ray attenuation values xM1 and xM2 at the different X-ray energies E1, E2 are known and can, for example, be gathered from a table. The same holds true for the X-ray attenuation value xKM of the substance to be determined. Said attenuation value can, if need be, also be determined in advance by a separate calibration measurement, for example by using a water phantom.
The present method and the associated device utilize the recognition that in reality many materials only occur in the human and animal body with an approximately constant density. Using this property as a starting point, this means that, in a CT image, even mixtures of two materials do not have arbitrary X-ray attenuation values. This was able to be verified experimentally for liver tissue. The CT value of liver tissue decreases linearly with an increasing proportion of stored fat. It is also known that the difference between the X-ray attenuation values at different tube voltages of the computed tomography scanner, i.e. at different X-ray energies, is a linear function of the fat content. This relationship can also be extended to other body materials and is utilized in the present method and the associated device.
Although all of the abovementioned methods allow examinations to be performed at different times with a number of contrast agents, the amount of radiation dose used increases proportionally with the number of examinations and the expenditure of time increases correspondingly.